What is a Bill of Materials?

What is a Bill of Materials?

A bill of materials (BOM) provides a list of all the raw materials or components,sub-components, assemblies, and sub-assemblies required to build or repair a product or service. The document typically uses a hierarchical format showing the finished product at the highest level and the individual materials and components at the bottom level. It also includes the instructions for collecting and using the materials.

The BOM document is a centralized source of information used to manufacture a product. The assembly process always begins with the creation of a BOM. Developing an accurate BOM document is essential because the correct parts have to be available when the item is built. If inaccuracies are present, the product can be delayed or completely halted, increasing optional costs due to the fact that the missing parts must be located, another production order must be started, or customers begin to return the product.

“There are different bills of materials for engineering in the design process and for manufacturing in the assembly process.”

Types of BOMs

Engineering Bill of Materials (EBOM)

The engineering BOM is a subset of a BOM that defines a product as designed, used in the design phase. It contains the list of items and assemblies in the product designed by engineering. Engineers usually handle this based on computer-aided design (CAD) or Electronic Design Automation (EDA) tools they use. One product design may include multiple EBOMs. For instance, the BOM for a printed circuit board assembly that’s been designed by an electrical engineer will list the resistors, chips, and capacitors. The BOM for a product that’s designed by a mechanical engineer on the other hand, will feature a list of purchased hardware (buttons, screws, etc.) and custom parts. No matter how many there are, all EBOMs deal with how a product is designed.

Manufacturing Bill of Materials (MBOM)

The manufacturing BOM is a subset of a BOM that defines how a product is assembled. It contains the details of all the parts required to build a product that you can ship to customers. If the MBOM isn’t accurate, you risk building the product too slowly, incorrectly, or being unable to build it at all.

The information from this document drives everything about it – the manufacturing, operations, purchasing, and logistics. It feeds business systems used to order parts and build the product, including enterprise resource planning (ERP), materials resource planning (MRP), and manufacturing execution system (MES) options.

The MBOM includes packaging materials, usage instructions (setup guides for the end customer) and all other items it takes to build the final shippable product. It also goes into detail about the items used in the assembly process include tape or liquid adhesives. It will list any off-the-shelf components, as well any made-to-specification parts. It will also include any non-tangible parts, such as firmware. Anything that can be found in the final boxed product as it arrives to the customer should be included on the MBOM at some level.

Some areas will require processing before they are ready to be assembled into the final product – such as programming or painting. The altered part is the only one that makes into the final product, but the pre-processed base, and the finished part must both be represented on the MBOM. This way, your manufacturing team can decide which steps should be taken care of in-house and which steps can be outsourced to another vendor. Over the life of the product, processing locations can be changed to improve quality, increase flexibility, or if necessary, reduce costs.

What Your BOM Should Include

All lines of the BOM will include the product code, part name and part number, part revision, a description, quantity, unit of measure, size, length, weight, and specifications or product features.

The BOM list is necessary when it’s time to order replacement parts as it reduces possible issues if product repairs are needed. It helps plan for acquisition orders and reduces the likelihood of error.

Your BOMs must be revisited regularly to ensure they are organized, correct, and current. This is especially true for companies that outsource any manufacturing, because inaccuracies in the BOM that’s handed to contract manufacturers or suppliers could result in production delays.

If you order the wrong parts or the wrong quantity of parts, you risk either not being able to build enough product, or not being able to build the product at all. This leaves your organization with unusable components that must be returned, or with extra parts that tie up capital in inventory. If you’re already running lean, cleaning up the mess is a hassle and a waste of both time and money. If the size of the original mistake is big enough, it could have an effect on your organization’s bottom line.

The more detailed your MBOM, the better. Your new product introduction (NPI) process will run smoother. You’ll have a more controlled ramp up to full production. You’ll keep things running smoothly in terms of resource management and procurement.

PurchaseControl can make it easier to stay on top of BOMs and order only what you need, when you need it.